Urological stent therapy system and method

ABSTRACT

A stent system and method for use in the prostate gland. The stent is made of a shape memory alloy such as nitinol, and has a low temperature martensite state, with a martensite transition temperature below body temperature, and a high temperature austenite state, with an austenite transition temperature at or above body temperature, and a memorized shape in the high temperature austenite state which is a helical coil of diameter large enough to hold the prostatic urethra open. The stent is used to heat the prostate and is left in the prostatic urethra while the prostate heals. After the prostate is substantially healed, the stent is cooled to its martensite state and is easily removed from the urethra.

This application is a continuation of U.S. application Ser. No.08/629,650 filed Apr. 9, 1996, now U.S. Pat. No. 5,830,179.

FIELD OF THE INVENTION

This invention relates to stents, urology, and treatments for benignprostate hypertrophy or prostate cancer, as well as methods forcorrection of vessel occlusions.

BACKGROUND OF THE INVENTION

The inventions described below were developed to aid in the treatment ofprostate enlargement and/or prostate cancer. Prostate enlargement, alsoknown as benign prostate hyperplasia or benign prostate hypertrophy, isa common affliction among older men. The condition involves swelling ofthe prostate. The prostate surrounds the urethra, or urinary tract, andswelling of the prostate prevents passage of urine from the bladder.Benign prostate hyperplasia is uncomfortable because it makes urinationdifficult or impossible, The condition is also dangerous because it canlead to infection of the bladder and kidneys, and severe cases may leadto death.

Prostate cancer is also a common affliction among older men, and maylead to many of the same symptoms as benign prostate enlargement.Prostate cancer is more dangerous in that it may spread to other organsand is often fatal. Early treatment can reduce the risks of death due toprostate cancer.

A surgical cure for prostate enlargement is called resection. Resectioncan be accomplished by cutting away a large portion of the prostategland. The operation can be performed by cutting through the skin toexpose the prostate gland, and using scalpels to cut into the prostate.Preferably, resection is accomplished from inside the urethra, using aresectoscope inserted through the penis. The resectoscope includes anendoscope for visual observation and a resecting loop which a surgeonuses to scrape and gouge away the prostate gland from the inside.

Prostate enlargement can be treated with heat treatments such ashyperthermia or thermotherapy, cold treatment (hypothermia orcryotherapy), and ablation. It has long been known that heating aswollen prostate gland can lead to a decrease in swelling and eventualrelief from the condition. Heat treatment denaturizes the proteins inthe prostate tissue, like a slow cooking of the tissues. The biologicaleffects of heat treatment and the appropriate thermal dosage arediscussed in more detail in articles such as Terai, et al.,Transurethral Microwave Thermotherapy For Benign Prostatic Hyperplasia,International Journal of Urology 24 (March 1995) and Pow-Sang, et al.,Thermocoagulation Effect Of Diode Laser Radiation In The Human Prostate,45 Urology 790 (May 1995), but it is sufficient for the purposes of thisdisclosure to understand that application of heat at sufficiently hightemperature for sufficient lengths of time to destroy some or all cellsin a portion of the prostate gland eventually produces a therapeuticeffect.

Devices for heating the prostate are illustrated, for example, inEdwards, et al., Medical Probe Device and Method, U.S. Pat. No.5,366,490 (Nov. 22, 1994), which shows a device for application of RF ormicrowave energy into the prostate while protecting the prostaticurethra from damage during the treatment. Hyperthermia treatment, as theterm is generally used, is accomplished in the temperature range of40-60° C. Thermotherapy, as the term is generally used, is accomplishedby heating the prostate above 60° C. Both heat treatments have beenbeneficially used in the treatment of prostate enlargement.

After heat treatment, the prostate gland will be partially destroyed.Thermal necrosis, thermocoagulation, denaturization, and other suchterms are used to describe the thermal damage done to the prostategland. The prostatic urethra will also be partially destroyed. Theprostate gland and the prostatic urethra swell in response to the burncaused by the heat treatment, and this immediately causes acute blockageof the urethra. The prostate gland and prostatic urethra eventuallyheal, over several weeks or months, typically about three months afterheat treatment.

During the healing period, much of the prostate and prostatic urethrathat were damaged by the heat treatment are re-absorbed by the bodythrough the blood vessels supplying the area. However, significantportions near the urethra slough off the urethra wall and fall into theurethra. Sloughing causes acute blockage of the urethra. Thus, duringthe post-operative healing period, swelling and sloughing cause acuteblockage of the urethra, leading to extreme discomfort and clinicaldanger to the patient. After healing, the prostate will be smaller thanbefore heat treatment and will not force closure of the urethra. Thecondition of benign prostate hyperplasia is essentially cured. Prostatecancer can also be treated successfully with similar heat treatments,usually in combination with chemotherapy or radiation treatment.

It has recently been proposed to use stents to support the urethra andkeep it open despite pressure from the swollen prostate. The Prostacoil™temporary intraprostatic stent, marketed by Instent, Inc. of EdenPrairie, Min., is an example of a stent adapted for use in the prostaticurethra. The stent includes an anchoring section and a prostaticsection, and is placed with a delivery catheter shaft through theurethra. The stent is used long-term, for patients temporarily orpermanently unfit for surgery.

A wide variety of stents have been proposed for use in variousapplications. Intravascular stents and coronary stents such as thePalmaz-Schatz stent illustrated in Palmaz, have been used to treatocclusions of blood vessels. A commonly suggested material for makingstents is pseudoelastic and/or shape memory alloys such as Nitinol. Forexample, Sugita, Catheter, U.S. Pat. No. 4,969,890 (Nov. 13, 1990)proposes use of a shape memory alloy for an intravascular stent, andshows a device for percutaneous delivery of the stent to an occludedstenotic region of a blood vessel. Harada, et al, Method of Implanting aStent Within a Tubular Organ of a Living Body and of removing Same, U.S.Pat. No. 5,037,427 (Aug. 4, 1991) proposes use of a two-way shape memoryalloy stent in a blood vessel. Two-way shape memory is useful in astent, according to Harada, to allow removal of the stent. As explainedin Harada, it is not possible to remove a one-way shape memory stentafter implantation. Harada proposes use of two-way shape memory stentwith a hot, large diameter shape which holds a blood vessel open and acold, small diameter shape which can be moved within the vessel andremoved. Harada also discloses a device for percutaneous placement ofthe stent. Dotter, Transluminally Placed Expandable Graft Prosthesis,U.S. Pat. No. 4,503,569 shows the use of shape memory alloy stentproposed for use in blood vessels. Each of these references use salinesolution injected through a catheter to control the temperature of thestent, thereby controlling the shape of the stent.

Stents may be left in blood vessels permanently, and are usuallyimplanted for permanent use. The risk of infection around the stent in ablood vessel, or movement of the stent within a blood vessel, aresomewhat limited by the environment. In the urethra, however, the riskof infection is high, and movement within the urethra may be caused byurination or ejaculation, especially if the prostate gland shrinks inresponse to treatment. Thus, there is a limit to the amount of time astent may be left implanted in the urethra before infection sets in ormigration occurs.

SUMMARY OF THE INVENTION

The devices described below include urological stents and devices forplacing the stents in the urethra. Methods for treating benign prostatehyperplasia or prostate cancer with heat treatment, either hyperthermiaor thermotherapy, using the stent as the heat source, are alsodescribed. Also, fabrication of the stent from a nitinol alloy, shapememory alloy, or pseudoelastic alloy, permits easy placement andsubsequent removal of the stent, so that the stent may be placed in theurethra during the healing period and removed when no longer necessary.The inventions disclosed and claimed below combine various aspects oftreatments discussed above and various new concepts to create newdevices and methods for treating benign prostate hyperplasia.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of the prostate gland with a stent inplace.

FIGS. 2 through 4 are cross sectional views of an enlarged prostateillustrating use of the stent.

FIG. 5 is cross-sectional view of lower abdominal portion of the humanbody with a stent in place.

FIG. 6 is a cross-sectional view of the prostate gland with a stent inplace and ready for removal.

FIGS. 7 and 8 are elevational views of the stent, with additionalfeatures for use within the prostatic urethra.

FIGS. 9 and 10 are elevational views of the stent delivery catheter foruse with the present invention.

FIG. 11 is a graphical illustration of the stent's behavior in responseto temperature changes.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a stent designed for use in the treatment of benignprostate hyperplasia or prostate cancer. The details of the localanatomy shown in this figure include the prostate gland 1, the urethra 2and the prostatic urethra 3. The urethra is the channel which conductsurine from the bladder 4 to the penis for discharge from the body. Theprostatic urethra is a continuation of the urethra, and it joins theprostate gland to the urethra. The boundary between the prostate glandand the prostatic urethra is ill defined, represented by the dashed line5. The bladder neck sphincter 6 controls flow of urine from the bladder4, and the external sphincter 7 controls flow of urine or ejaculate fromthe bladder 4 or prostate 1. The prostate capsule 8 surrounds theprostate gland. The prostate gland consists of various tissues,including glandular tissue (which produces ejaculate), muscular cells,and epithelial cells. The inside diameter of urethra 2 is typicallyabout 2 centimeters, and the prostatic urethra varies in length fromabout 15 to 75 mm.

The condition of benign prostate hyperplasia causes the prostate toswell and close off the prostatic urethra, as illustrated in FIG. 2. Theurethra 2 is squeezed shut by the swollen prostate, and has an occludedregion 9. The stent 10 mounted on the distal portion of deliverycatheter 11 with an atraumatic tip 12 is ready for placement in theoccluded portion of the prostatic urethra. The stent is made of anitinol alloy with a martensite transition temperature slightly belowbody temperature, in the range of 30-35° C. (this range is convenientlyestablished or maintained with cold saline flow through the catheter ora catheter sheath). Thus, when the stent is cooled below bodytemperature by cold saline flow, it will enter the soft and pliablemartensite state of the alloy. The chosen alloy has a wide hysteresis,so that it remains in the soft and pliable martensite state for atemperature range distinctly above the temperature at which it convertsto martensite upon cooling. The transition temperature for the change tothe austenitic state upon heating is slightly above body temperature, inthe range of 38-60° C. or even higher, depending on the heating sourceused. When hot saline solution is used, 38-60° C. is convenient becausethat temperature range can be easily achieved by flushing hot salinethrough the catheter into the vicinity of the stent (100° C. is theequivalent to 212° F., the boiling point of water, so it can beappreciated that the temperature range or 38-60° C. is easily achievedin the operating room).

FIG. 2 depicts placement of the stent. The stent pictured in FIG. 2 hasbeen cooled to accomplish the martensite transition, and the stent issoft and pliable. The stent is tightly wound around delivery catheter11, and has a small diameter of about 1 cm that fits easily into theurethra 2. The catheter sheath 13 is provided to cover the stent duringplacement and provide a smooth outer surface to facilitate placement ofthe stent. The stent 10 is then pushed into the occluded region 9 of theprostatic urethra 3, as shown in FIG. 3. Preferably, the stent has anaustenite transition temperature above body temperature, and there is noneed to flush saline through the delivery catheter or sheath to keep thestent cooled below body temperature. Where the austenite transitiontemperature is at or below body temperature, the stent should be cooledwith cold saline flow to maintain its martensite state until it isproperly placed. Alternatively, the stent can be pseudoelastic orsuperelastic at body temperature, and in this case may be held in itssmall diameter shape by catheter sheath 13, in which case the sheathserves as a retaining mechanism for the stent.

After placement in a swollen prostate, as depicted in FIG. 3, the stentwill be firmly held by the compressive forces of the prostate ifswelling is severe enough, which it usually is in cases warrantingintervention. Either during or after placement, the stent is flushedwith hot saline which causes the stent to heat up above its austenitetransition temperature. Of course, if the stent transition temperatureis at or below body temperature, it will be sufficient to allow thestent to be heated to the austenite transition temperature bysurrounding body temperature without injection of warm saline solution.Upon this transition, the stent recovers its original large diametershape and forces the prostatic urethra open, as shown in FIG. 4. Thestent may be left in the urethra for some time, but eventual infectionis almost certain, so that heat therapy is accomplished according to thefollowing description to ensure that the stent is needed for only ashort period of time.

To heat the prostate, the stent is used as the heat source. Preferably,radiofrequency energy (RF energy) is broadcast from outside the body,from RF transmitter 14 as illustrated in FIG. 5. RF transmittersavailable from valley Labs of Boulder, Colo. capable of transmitting RFenergy at powers of up to approximately 300 watts are sufficientlypowerful to heat the stent and cause the stent reactively radiate RFenergy into the prostate. The hot stent radiates heat into the prostate,and to some extent the reactive radiation from the stent heats prostatetissue. Radiation is maintained for an adequate length of time to heat alarge portion of the prostate, indicated by dashed line 15 shown in FIG.2, to 60° C. or more. FIG. 4 shows dashed line 15, which represents athermocline of 60° C., and other isotherms of 70° C. (dashed line 16)and 80° C. (dashed line 17 ) are shown to illustrate the expectedtemperature gradient within the prostate during the heat therapy.Typical therapeutic treatments require approximately 10 to 40 minuteswith the stent irradiated by 20 to 40 watts of RF energy to createtemperatures of at least 40° C. (and preferably 60° C.) in the region tobe destroyed. The heating may cause damage to a substantial portion ofthe prostate, but should avoid damage to surrounding tissues such as theneurovascular bundle 18 shown in FIGS. 1 and 2 and the colon 19 shown inFIG. 5.

During radiation, the stent may be grounded by attaching a ground wire20 to the stent through the urethra. The ground wire is carried on thedelivery catheter, and is releasably attached to the stent or merely incontact with the stent, so that the ground wire may be withdrawn whilethe stent is left in place. Alternatively, a grounding pad 21 can beplaced on the skin near the prostate, providing a ground path throughthe body to the pad. The heating of the prostate can be monitored withone or more temperature probes 22 inserted through the skin in thetransperineal area between the scrotum and the anus. Alternatively,heating may observed via ultrasound imaging through a transrectalultrasound probe 23. The appearance of healthy and heat damaged prostatetissue may be differentiated on ultrasound images obtained through theultrasound probe, and the progress of heat therapy can be monitored onthe ultrasound displays provided by the ultrasound imaging systemassociated with the probe. The surgeon performing the therapy willdecide when a sufficient portion of the prostate gland has been heatdamaged so that eventual re-absorption of that damaged tissue willresult in the cure of the condition. As an alternate to directobservation, surgeons may establish standard dosages of RF energy, interms of wattage, frequency, and time, which ensure adequate heattreatment without danger of damage to surrounding organs, and may beapplied without need to monitor thermal damage directly.

Alternate heating means may be employed. For example, the stent may beconnected to a DC, AC or radiofrequency source through electricalconnections running from the stent, through the delivery catheter, to anexternal power source. These sources can be used to heat the stent totemperatures sufficient for heat therapy. The same sources may be usedfor heating the stent to its shape recovery temperature.

After heating via the stent, the heated portion of the prostate glandswells and dies off. The prostatic urethra is also damaged by thetreatment and swells and dies. Further occlusion of the urethra isprevented by the stent, which is left in place for some time after heattherapy. While the body's waste removal mechanisms re-absorb thedestroyed prostate cells, the stent holds the urethra open and preventssloughing of dead tissue into the urethra. Over several months, the bodywill re-absorb the portions of the prostate gland and prostatic urethradamaged by the heat therapy, and the enlargement will subside. Duringthis healing time, the patient has benefited from the stent because ithelps avoid the short term closure usually associated with heat therapy.

As the enlargement subsides, the prostate and prostatic urethra willsubside from the stent, and the stent will become loose within urethra.The stent also presents an infection site, and if left in permanentlywill probably cause infection within the urethra. Thus, when theprostate gland has receded enough that the urethra will be patent uponremoval of the stent, and before infection is likely to set in, thestent should be removed. The expected time frame for removal of thestent is two to three months after placement and heat therapy.

Removal of the stent is accomplished by inserting a catheter andflushing the stent with cold saline solution to cause reversion of thestent into the soft, pliable martensite state. As illustrated in FIG. 6,an endoscope 24 and endoscopic graspers 25 are them inserted through acatheter 26 to find and remove the stent. Because the stent is now softand pliable, typically as easily deformable as silver solder, the stentcan be grasped at its proximal end and pulled through the catheter. Asthe stent is pulled through the catheter, it unravels and deforms toeasily fit through the catheter. Alternatively, the grasper can be usedto hold the stent in place while the sheath 27 is gently pushed over thestent. The cold stent is soft and pliable and offers little resistanceto the sheath.

When the stent is removed, the urethra is open, and the prostate isalmost entirely healed. There may be some residual dead tissuethroughout the prostate and the prostatic urethra, but this dead tissuewill be removed without incident by the body, either through sloughing,which should occur without ill-effect through the now open urethra, orthrough continued re-absorption. At this stage, the condition of benignprostate enlargement has been cured.

A number of other features are added to the stent in order to optimizeits use within the prostate, as illustrated in FIG. 7. Althoughdestruction of the prostate gland is the desired goal, destruction ofsurrounding structures is undesirable. The bladder neck sphincter 6 andthe external sphincter 7 control urine flow, and damage to thesestructures would lead to incontinence. To avoid thermal damage to thebladder neck sphincter and the external sphincter, the proximal end 28and distal end 29 of the stent shown in FIG. 7 are thermally insulatedto limit the heat transfer from the stent to the prostate gland at thedistal and proximal ends of the stent. Insulating layers of PTFE(Teflon®) or other suitable plastics are sufficient to protect thesestructures.

The possibility of stent migration can be limited by providing flaredends on the stent. Flared ends 30 and 31 on the stent help to anchor itto the prostate gland and prevent slippage and migration of the stent. Asmall knob 32 is provided on the proximal end of the stent, attached tothe end of the ribbon, and this provides for easier grasping of thestent when it is removed. An optional anchor section 33 and retainingwire 34 can be added to the stent, as shown in FIG. 8. The anchorsection is spaced from the stent so that it resided downstream in theurethra and does not obstruct the external sphincter and is not subjectto the RF energy used to heat the stent itself. The anchor section mayalso be thermally and electrically insulated to protect the urethra fromany heat that might be conducted from the stent or caused by accidentalirradiation from the RF source. The electrical and/or thermal insulationis shown in FIGS. 7 and 8 as the lightly cross hatched areas at eitherend of the stent.

Typical sizes for the stent are shown in FIG. 7 also. The stent isintended for use in the prostatic urethra 3, which varies in size fromman to man. The length of the straight segment 35, when expanded, isabout 3-5 cm. The diameter of the straight segment 35, when expanded, isabout 2 cm. The length of the flared end portions is about 2 cm, with adiameter of 3 cm. Thus the overall length or the stent is about 7 to 9cm. It may prove desirable to manufacture the stents in just a fewstandard sizes corresponding to commonly encountered prostate sizes,insofar as an exact size match is not usually necessary. If anon-standard size or an exact size is desired, stents may be speciallymanufactured to specification. Thus, variations of size beyond the sizesmentioned here may occasionally be needed and accomplished.

The stent will be inserted in its cold state, while it is pliable andeasily deformed so that it may be tightly wrapped around the deliverycatheter. The catheter diameter may be as small as the state of the artallows, but need only be small enough to fit comfortably into theurethra. Delivery catheter diameter of 1-1.5 cm with a stent tightlywound around the catheter is sufficiently small to allow easy accesswith standard catheter insertion techniques. The stent illustrated inthe drawings is a ribbon coiled stent. When heated to its hightemperature state, the stent takes on the form of a helical coil of flatwire or ribbon, with the successive coils closely spaced, perhapsactually touching, to prohibit intrusion of swollen prostate tissue orsloughing tissue into the prostatic urethra. Closely spaced round wirecoils may also be used, and other stent configurations such as expandedmetal stents, braided stents and others may also be used.

FIG. 9 illustrates a delivery catheter 11 with the stent 10 tightlywound about the catheter shaft. The catheter is a multi-lumen catheter,with lumens 36a, 36b, 36c, etc. communicating from the proximal end ofthe catheter to the distal end, with distal outlets 37a, 37b and 37c,etc.. With the stent in place around the catheter in the stent recess38, which is a length of reduced cross section on the catheter betweendistal retaining shoulder 39 and proximal retaining shoulder 40. Forcedflow of warm saline solution out the distal outlets 37 of lumens 36 willwarm the stent and cause the distal end of the stent to change to itshigh temperature memorized shape. As the distal end expands, it willexpand against the swollen prostate, and serve to anchor the distal endof the stent, ensuring proper placement of the entire stent whenever thedistal end is properly placed. As water diffuses in the prostaticurethra, the more proximal areas of the stent will heat up and revert tothe memorized shape, and the stent will expand radially, starting at thedistal end and progressing to the proximal end, and thereby anchoritself into the prostatic urethra.

As illustrated in FIG. 10, the injection of saline may also beaccomplished proximal end first, by providing additional lumens 41a,41b, 41c, etc. and lumen outlets 42a, 42b, 42c, etc. in the distal endof the full diameter portion of the delivery catheter. These lumens willprovide saline flow to the proximal end of the stent, so that theproximal end will be warmed first, expand and engage the prostate, andanchor the stent from the distal end. (Where an anchor section isprovided, the anchor section may be deployed first). In either distalfirst or proximal first warming, once the end of the stent has expandedso that its diameter is larger than distal retaining shoulder 39 orproximal retaining shoulder 40, the delivery catheter can be movedproximally or distally to place the lumen outlets near unheated portionsof the stent, thereby controlling expansion of the stent.

Various clinical factors, including successful visualization of theprostatic urethra, will determine whether a surgeon decides to employproximal first or distal first deployment. The device may bemanufactured to allow for one or the other, or for both simultaneously,or allow selective flow to outlets 37 or 42 during the deploymentprocedure. In FIG. 9, the proximal end 43 of the delivery catheter isfitted with a proximal hub 44 and a luer fitting 45, which provide aninput port for saline solution to lumens 36. In FIG. 10, the hub isprovide with an additional luer fitting 46 which provides saline flow tothe additional lumens which communicate with a coaxial chamber in thehub communicating with proximal outlets of the lumens 41. Where it isdesired to provide simultaneous flow paths to both the proximal outletsand the distal outlets, an additional luer fitting is unnecessary and asingle chamber within the hub communicating with both sets of lumens 36and 41 can be provided.

Catheter sheath 13 facilitates placement of the stent. The sheath istransparent and flexible. It is placed inside the urethra over anendoscope. The distal marking 47 is readily visible through theendoscope, as are ruled markings 48. The scope is inserted until thebladder neck sphincter is seen through the scope, and then the cathetersheath is advanced over the scope until the distal marking is visibleand placed near the bladder neck sphincter. The endoscope is then pulledback until the external sphincter is located, and the ruled markings onthe catheter sheath are used to measure the prostate gland so that astent of appropriate size can be selected for use. The catheter sheathis left in place while the delivery catheter with a stent tightly woundupon the stent recess is inserted into the urethra through the cathetersheath. An endoscope is inserted either side-by-side with the stent, orwithin the stent as part of the delivery catheter. When the distal endof the stent is aligned with the desired point of deployment, justinside the prostate, downstream from the bladder neck sphincter, warmsaline can be injected to cause expansion and deployment of the distalend of the stent.

FIG. 11 illustrates the metallurgical behavior of the stent. The stentis made of a shape memory alloy with a martensite state at coldtemperature and an austenite state at high temperature, as ischaracteristic. Nitinol, comprised mostly of nickel and titanium, andusually alloyed with various other metals, is the most common shapememory alloy, however numerous alloys behave in similar fashion. At lowtemperature, the stent is in its martensite state, and is very pliableand has no memorized shape (except in alloys exhibiting two-way shapememory) and has very little strength. This is shown on the graph oncurve A. As temperature rises, at a certain temperature (determined by avariety of factors, including composition of the alloy, readilycontrolled in the art of shape memory alloys) called the austenite starttemperature, T_(as), the metal starts to convert to austenite. The metalbecomes stronger, stiffer, and reverts to its memorized shape astemperature increases to T_(af). At the austenite finish temperature,T_(af), the alloy has completely reverted to austenite, has recoveredits memorized shape (unless restrained), and is stiff like spring steel.Above T_(af), temperature increases do not affect the behavior of themetal (although it may distemper the metal and destroy its memory). Uponcooling, the metal reverts to the martensite state, but this does notoccur exactly in reverse. The temperature at which reversion tomartensite occurs upon cooling is lower than the temperature at whichmartensite-to-austenite conversion occurs. As shown in the graph, uponcooling to the martensite start temperature, T_(ms), which is below bodytemperature, the metal start to become pliable. Further cooling to themartensite finish temperature T_(mf) results in the complete conversionof the alloy to the soft, pliable martensite state. In the region oncurve B above T_(ms), the metal remains stiff and strong, even thoughthe metal was soft and pliable at the same temperature upon heating onCurve A. Thus the alloy must be cooled considerably below the shapememory transition temperature T_(af) before reversion back to the softmartensite state. The behavior is referred to as hysteresis, which is aterm which generally refers to delays in changes of states of varioussystems. In some alloys, superelastic behavior occurs around the regionof Curve B above T_(ms). In this region, the metal may be substantiallybent (deformed) but still spring back to its memorized shape, and thisbehavior is markedly different than normal metals. This region is shownon the graph as T_(sim), which varies from alloy to alloy and might notbe present in some alloys. Alloys and devices incorporating thesecharacteristics may be manufactured according to known methods in theart of metallurgy.

These characteristics may be employed in the stent as follows. The stentis preferably made with an upward transition temperature (either T_(as)or T_(af)) slightly above body temperature. The stent is also made witha downward transition temperature (either T_(ms) or T_(mf)) below bodytemperature. The stent is trained using known techniques to memorize thehigh temperature shape illustrated above. Thus, once heated to cause thehigh temperature state with its memorized shape, the stent will remainin that shape at body temperature, and will hold the prostatic urethraopen indefinitely. When it is desired to move the stent, cooling thestent with cold saline flow to a temperature below Tms causes it torevert to the cold pliable state, allowing easy deformation so that thestent may be pulled out of the urethra and into a catheter sheath. Inalternate embodiments, superelasticity may be incorporated into thestent, so that if the stent is superelastic at body temperature, it maybe restrained by the catheter sheath, inserted into the urethra andreleased by pulling the catheter away from the stent, allowingsuperelastic reversion to the memorized state. Two-way shape memorystents may also be used, such as those described in Harada, wherein thestent is fabricated to have a memorized shape at high temperature and atlow temperature, wherein injection of cold saline around the stentcauses reversion to a low temperature memory shape with a smalldiameter. However, given the expected shrinkage of the prostate gland inresponse to therapy, the prostate will already be receding from thestent, and it is possible to remove a one-way memory stent in thisenvironment. Thus, two-way shape memory is not necessary in the normalprostatic treatment case.

While the preferred embodiments of the devices and methods have beendescribed, they are merely illustrative of the principles of theinvention. Other embodiments and configurations may be devised withoutdeparting from the spirit of the inventions and the scope of theappended claims. While the inventions have been described in theenvironment of urology, the application of the inventive concepts inother areas of heat therapy and temporary stent application will bebeneficial. It is specifically contemplated that the materials employedin the illustrated embodiments be improved upon, that methods andpatterns of irradiation and heating of the stent be improved upon, andthat the time tables for therapy discussed above be improved upon, andthat all such improvements fall within the scope of the claims.

We claim:
 1. A stent for use within the prostatic urethra, said stentcomprising:a length of flat wire comprised of a shape memory metalhaving a high temperature state in which the shape memory metal istrained to revert to the shape of a helical coil, and a low temperaturestate in which the metal is easily deformable; wherein said lowtemperature state occurs when the stent is cooled to temperatures belowbody temperature, and said low temperature state is retained uponheating to body temperature, and the high temperature state occurs whenthe stent is heated to temperatures above body temperature and said hightemperature state is retained upon cooling to body temperature.
 2. Astent for use within the prostatic urethra, said stent comprising:alength of flat wire comprised of a shape memory metal having a hightemperature austenite state in which the shape memory metal is trainedto revert to the shape of a helical coil, and a low temperaturemartensite state in which the metal is easily deformable; wherein saidlow temperature martensite state occurs when the stent is cooled totemperatures below body temperature, and said low temperature martensitestate is retained upon heating to body temperature, and the hightemperature austenite state occurs when the stent is heated totemperatures above body temperature and said high temperature austenitestate is retained upon cooling to body temperature.
 3. The device ofclaim 2 wherein the high temperature state is achieved upon heating to atemperature range of about 38-60° C., and the low temperature martensitestate is achieved upon cooling to the temperature range of 30-35° C.